Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-12-26
2003-07-01
Egwim, Kelechi (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S289000, C524S291000, C524S303000, C524S304000, C524S392000, C524S585000
Reexamination Certificate
active
06586509
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composition for electric cables, more particularly a composition for an insulating layer of an electric cable, preferably a medium- or high-voltage electric cable. The composition contains an ethylene plastic and additives including a peroxide cross-linking agent and antioxidant additives.
BACKGROUND OF THE INVENTION
Electric cables and particularly electric power cables for medium and high voltages may be composed of a plurality of polymer layers extruded around the electric conductor. In power cables the electric conductor is usually coated first with an inner semiconductor layer followed by an insulating layer, then an outer semiconductor layer followed by water barrier layers, if any, and on the outside a sheath layer. The layers of the cable are based on different types of ethylene plastics which usually are crosslinked.
The insulating layer of an electric cable is composed of ethylene plastic. By the expression “ethylene plastic” is meant, generally and in connection with the present invention, a plastic based on polyethylene or a copolymer of ethylene, wherein the ethylene monomer constitutes the major part of the mass. Thus, polyethylene plastics may be composed of homopolymers or copolymers of ethylene, wherein the copolymers may be graft copolymers or copolymers of ethylene and one or more monomers which are copolymerisable with ethylene. LDPE (low-density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure) is today the predominant cable insulating material. As mentioned above the ethylene plastic may also be an ethylene copolymer, and in that case it includes from 0 to about 25% by weight, preferably about 1-15% by weight of one or more comonomers which are copolymerisable with ethylene. Such monomers are well known to those skilled in the art and no extensive enumeration will be required, but as examples, mention can be made of vinylically unsaturated monomers, such as C
3
-C
8
alpha olefins, for instance propene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene; C
8
-C
14
non-conjugated dienes, for instance 1,8-octadiene and 1,10-decadiene; and vinylically unsaturated monomers containing functional groups, such as hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. Such monomers may comprise, for instance, (meth)acrylic acid and alkyl esters thereof, such as methyl-, ethyl- and butyl(meth)acrylate; vinylically unsaturated, hydrolysable silane compounds, such as vinyl trimethoxy-silane; vinyl acetate etc. However, if the ethylene plastic is an ethylene copolymer the amount of polar comonomer should be kept at a minimum such that the polar comonomer comprises at most 10% by weight of the total ethylene plastic composition. Besides the additives described in more detail below, the remainder of the composition according to the present invention is made up of one or more of the ethylene plastics specified above. This means that the amount of ethylene plastic in the composition should lie in the range from about 95% by weight to about 98.8% by weight of the composition.
In order to improve the physical properties of the insulating layer of the electric cable and to increase its resistance to the influence of different environmental conditions, the ethylene plastic contains additives the total amount of which usually is about 0.2-5% by weight, preferably about 0.3-4% by weight. These additives include stabilising additives such as anti-oxidants to counteract decomposition due to oxidation, radiation, etc.; lubricating additives, such as stearic acid; additives for water-tree resistance, such as polyethylene glycol, silicones, polyglyceryl esters etc.; and crosslinking additives such as peroxides which decompose upon heating and initiate crosslinking of the ethylene plastic of the insulating composition, optionally used in combination with unsaturated compounds having the ability to form crosslinks when initiated by radical forming agents.
There is a large number of different additives and the number of possible combinations thereof is almost unlimited. When selecting an additive for a polymer composition the aim is to optimise the positive properties as much as possible, while any negative properties should be minimised. However, it is usually very difficult or even impossible to completely eliminate the negative properties that may accompany an additive. It must therefore be regarded as considerable technical progress if, in view of the number of different additives and the possible combinations thereof, a combination could be found which shows the advantages of prior-art additives without any of their disadvantages.
Known types of antioxidants are sterically hindered phenols, aromatic amines, organic phosphites, and thio compounds.
One such known antioxidant for polyethylene and other plastic resins is 4,4′-thio-bis(6-tert-butyl-m-cresol) and has the formula (I):
This antioxidant can be obtained from several suppliers under different trademarks, one of which is SANTONOX® from Monsanto Co. The compound is known to be very efficient in improving the thermal ageing properties of cable insulation, and it is also known to be incorporated into the polymer network when crosslinking a composition with a peroxide of type dicumyl peroxide, thus reducing any risk for depletion of the antioxidant from the cable insulation during service. It also reduces the risk for premature crosslinking in a crosslinkable composition during extrusion. When used for insulating layers of electric cables (I) is usually added to the polyethylene base resin in an amount between about 0.1-0.5% by weight, depending on the degree of thermooxidative stress expected. In some cases the use of (I) may be accompanied by undesired side-effects, however, where higher loadings are required due to higher thermooxidative stress. Thus, since the solubility of (I) in polyethylene is limited, it tends at higher loadings to be partly sweated out, thereby generating dust on the surface of the pellets of the polymer composition to which it has been added. This dust may constitute a health hazard and is therefore negative from an environmental point of view. Further these dust particles of (I) have so high melting point that they do not melt at temperatures commonly used for simultaneous extrusion and crosslinking of polyethylene materials and may therefore appear in the extruded polymer insulation as particles or grains and make it inhomogeneous. In view of these disadvantages connected with (I) it has been tried to find other additives which can replace it without having the disadvantages thereof.
One commonly used replacement for the antioxidant (I) is a combination of the additives with formulae (II) and (III), respectively, as shown below:
C
18
H
37
—O—(C═O)—CH
2
CH
2
SCH
2
CH
2
—(C═O)—O—C
18
H
37
(II)
The compounds (II) and (III) are well known under the the tradenames IRGANOX® PS802 and IRGANOX® 1035, respectively. These antioxidants may be obtained from Ciba Specialty Chemicals. In HV-insulation compositions these two additives are used in an amount of about 0.1-0.5% by weight, preferably about 0.2% by weight of each in the polymer composition. Even though this additive combination eliminates some of the disadvantages of (I), it sometimes gives rise to undesired, premature crosslinking during extrusion of the polymer composition, which appears as so-called scorch, i.e. as inhomogeneity, surface unevenness and possible discoloration, in the insulation layer of the finished cable.
In an attempt to eliminate the scorch problem when using the antioxidant combination of (II) and (III) a third additive of the formula:
has been added. The compound (IV) can be obtained from Nippon Oils and Fats under the tradename Nofmer MSD. This compound serves both by converting highly reactive radicals formed by peroxide decomposition into less reactive ones, thereby reducing the scorch tendency, and by forming grafted bridges between polymer chains, thereby increasing the u
Boström Jan-Ove
Dammert Ruth
Gustafsson Bill
Smedberg Annika
Borealis Technology Oy
Egwim Kelechi
Merchant & Gould P.C.
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